Production of tubular products from metallic powders

ABSTRACT

Tubular-shaped powder metallurgical compacts are made by process comprising subjecting a mass of metallic powder confined in a tubular cavity to pressure directed outwardly from the interior of the cavity and progressing continuously from one end of the mass to the other.

United States Patent Charles E. Manilla Huntington, W. Va.; Franklin C. Kelly, Chesapeake, Ohio;

Inventors Richard H. Hanewald, Huntington, W. Va.

756,222 Aug. 29, 1968 Oct. 26, 1971 Appl. No. Filed Patented Assignee New York, N.Y.

The International Nickel Company, Inc.

PRODUCTION OF TUBULAR PRODUCTS FROM METALLIC POWDERS 13 Claims, 8 Drawing Figs.

U.S. Cl 75/214, 75/200, 264/111 Int. Cl B22f 3/12 Primary Examiner-Carl D. Quarforth Assistant Examiner-Brooks H. Hunt Attorney-Maurice L. Pinel ABSTRACT: Tubular-shaped powder metallurgical compacts are made by process comprising subjecting a mass of metallic powder confined in a tubular cavity to pressure directed outwardly from the interior of the cavity and progressing continuously from one end of the mass to the other.

PRODUCTION OF TUBULAR PRODUCTS FROM METALLIC POWDERS The present invention relates to powder metallurgy and more particularly to powder metallurgical processes for producing tubular-shaped products and to apparatus for producing powder metallurgical compacts.

It is well known that many articles have been made by powder metallurgical processes comprising pressing metallic powders together at room temperature with sufficient pressure to obtain cohesion of the particles and thereby provide a compact having sufficient strength to retain its shape when the pressure is released. Frequently the compact is sintered after pressing in.order to obtain improved density, uniformity and strength in the compact. In some instances it is desirable to use sintered compacts as metal stock for making wrought products. It is also known that powder metallurgical processes frequently enable obtaining a number of economic advantages such 'as elimination of needs for melting and casting metals into ingots and working ingots down to sizes near those required for making the finished articles. It is also understood that making articles from metallic powders instead of from melted alloys sometimes enables production of articles from metallic compositions containing ingredients that are difficult or impossible to combine by known melting practices, e.g., compositions comprising metal oxide powders and powders of elemental metals.

In spite of the known advantages offered by the potentials of powder metallurgy, presently known processes for compacting metal powders are not wholly satisfactory for making all the shapes and sizes in which articles are desired. Production of complex shapes, such as tubing and other tubular articles, from metallic powders presents hitherto unsolved problems involving compacting powder uniformly to obtain satisfactory uniform and high density in the compact. Uniform and high density of compaction are generally required where high strength is needed in the finished article. Moreover, even when a certain amount of porosity is desired in the finished article, e.g., in filters or bearings, uniformity of compaction is highly desirable in order to control the porosity. Control of compaction, of course, usually involves control over application and distribution of compacting pressure. In the powder metallurgical art it is understood that as a practical matter a mass of metallic powder under pressure in a mold (or die) does not behave like a true fluid in that the applied pressure is not transmitted uniformly throughout the mass. Failure of loose powder to behave like an ideal fluid is generally attributed to friction at the mold wall and to internal friction that occurs between particles of the powder. Although such friction and pressure distribution difficulties may be somewhat alleviated by use of lubricants, compaction of metal powders by usual methods of single-ended or double-ended pressing in a mold is generally unsatisfactory for producing uniform highdensity compacts when the configuration of the compact is such that the length of the compact along the direction in which it is pressed is greater than about 5 times the minimum cross-sectional dimension of the compact.

Heretofore, short tubular compacts, e.g., hollow cylindrical compacts having length-to-wall thickness ratios (L:T ratios) up to about 1:1, have been made by confining metal powder in an annulus between a hollow cylindrical mold and a concentrically disposed cylindrical core and then compressing the powder from one or both ends of the annulus. However, when attempting to make longer and thinner tubes it is generally found that pressing tubular compacts endwise in such a manner fails to produce uniform compaction if the L:T ratio of the tube form is greater than about 5:1. Moreover, for producing cylindrical tubular compacts having the uniformly high density that is needed in compacts which are to be sintered and cold-drawn to tubing, such single-ended and double ended pressing methods are wholly unsatisfactory for making long, relatively thin, tubular compacts with high L:T ratios of 5:! and higher.

It hus also been proposed to produce tubular compacts by isostatic pressing methods whereby a powder annulus is enclosed in a flexible envelope and pressure from a fluid is applied simultaneously at all points around the envelope inside and/or outside of the powder. The isostatic pressing fluid may be oil, water or gas. Although isostatic pressing in flexible envelopes may produce uniform compression of powders, such processes generally suffer from a number of disadvantages including difficulties in removing entrapped air and also difficulties in obtaining close dimensional tolerances due to flexure of the envelope during filling and pressing. Further, where it is required to produce long tubes of many feet in length, the need for obtaining sufficient force to provide the required pressure simultaneously over large areas of the tube gives rise to needs for undesirably large and expensive apparatus, especially when a high production rate is required. Moreover, the requirement for maintaining an impervious enveloping barrier between the powder and the pressing fluid presents a source of accidental failure in production inasmuch as even a small leak in the envelope can result in entrance of the fluid into the powder mass, thus spoiling the product.

Other known methods for compacting metallic powder include powder-rolling processes and stepwise intermittent compaction processes. Of course, these methods are obviously very difficult or are wholly impractical to apply to production of hollow, cylindrical articles. Moreover, intermittent stepwise compaction may lead to detrimental lack of uniformity in the product.

Although many attempts were made to overcome the foregoing difficulties and disadvantages and other difficulties, none, as far as we are aware, was entirely successful when carried into practice commercially on an industrial scale.

There has now been discovered a new process whereby metallic powders are compacted to provide tubular-shaped compacts having good uniformity of compaction with close control of dimensional requirements. Further, new apparatus having special advantages for pressing metallic powders to make long tubular compacts has also been discovered.

It is an object of the present invention to provide a process for producing a powder metallurgical compact having a tubular shape.

It is a further object of the invention to provide a process whereby a mass of metal powder can be compacted to a tubular shape with close control of the cross-sectional dimensions of the tube and with uniform high compaction of the powder in hollow cylindrical configurations having length-to-wall thickness ratios of about 5:1 and greater.

The invention also contemplates a process for production of compacted and sintered tubular products and a process for producing tubing by compaction, sintering and drawing.

Another object of the invention is to provide apparatus for making metallic powder compacts.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which:

FIGS 1 through 3 depict, in longitudinal partial cross section, stages of an illustrative process in accordance with the invention and an embodiment of apparatus in accordance with the invention;

FIG. 4 shows a perspective view of a tubular-shaped powder metallurgical compact produced in accordance with the invention;

FIGS. 5 through 7 illustrate additional embodiments of powder compaction apparatus in accordance with the invention; and

FIG. 8 depicts in cross section a process step of drawing a compact into a tube.

Generally speaking, the present invention contemplates a process for compacting a mass of metallic powder to a hollow tubular shape comprising filling a tubular cavity with a mass of metallic powder, thereby providing a tubular mass having an inner tubular surface and an external tubular surface, and then expanding the cross-sectional areas enclosed by the internal tubular surface continuously and progressively in a direction from one end of the mass to the other end of the mass while maintaining essentially constant the cross-sectional areas enclosed by the outer tubular surface and also maintaining the length of the powder mass essentially constant, to thereby compress and densify the powder mass and reduce the wall thickness thereof to produce a tubular compact. During expansion of the internal cross sections of the powder mass, pressure from a forwardly moving rigid surface is applied mainly outwardly and slightly forwardly against the powder. Advantageously, in progressively expanding the internal surface of the mass, a tapered mandrel is drawn small end first through the powder while it is confined in a mold so that compressive forces which are created by passage of the mandrel compact the powder between the mandrel and the inside surface of the mold. After compaction, the compact, sometimes referred to as green compact, is usually sintered to obtain greater density and strength and can then be hot and/or cold worked into wrought products. For instance, full density seamless tubing is made by powder compaction in accordance with the invention by sintering the green compact and cold forming the sintered compact using conventional tube forming apparatus, e.g., tube reducing and tube drawing.

The compact can be sintered in the mold in which it is made, in which event the sintering shrinkage will enable easy removal of the compact from the mold, or the compact can be taken out of the mold before sintering. To facilitate removing green compacts from the mold it is advantageous to have the mold split longitudinally into two or more parts so that the mold can be opened after compaction. More advantageously, a removable liner is provided in the mold. By employing liners of different thickness, the same mold shell can be used in making compacts of different outside dimensions. Also, a split liner is advantageous for facilitating removing and handling the compact.

I It is to be understood that the invention is generally satisfactory for compacting most metallic powder mixtures that are compactible at room temperature by other methods. The invention is particularly satisfactory for compacting ductile metal powders, such as nickel powder, cobalt powder, iron powder, copper powder, aluminum powder, magnesium powder, powders of nickel-copper alloys and powders of ductile nickel-chromium alloys, e.g., alloys containing I percent to 20 percent chromium with balance essentially nickel, and also powders and powder mixtures of metals and alloys having similar ductility characteristics. The metallic powder can comprise metal characteristics. The metallic powder can comprise metal oxides and other metallic compounds including thorium oxide, aluminum oxide, magnesium oxide, silicon carbide and tungsten carbide, e.g., a ductile metallic powder mixture can contain small amounts up to I percent of metallic compounds with the balance being essentially ductile metal powder, e.g., one or more ductile metal powders in proportions totaling at least 90 percent of the mixture. All compositional percentages set forth herein are by weight. Average particle sizes of powders compacted by the present process are usually from about 0.01 micron, or less, to about 200 microns, advantageously, about 1 micron to about 50 microns.

The invention is particularly successful in providing accurately dimensioned and uniformly dense compaction of powder into tubular forms having high L:T ratios of about 5:1 and greater, e.g., 250:1 and 350:1. The invention also has advantages of enabling practical production of very long tubular compacts, e.g., compacts 7 feet or feet or more in length. The range of wall thicknesses which can be compacted satisfactorily of course depends somewhat on the characteristics of the powder, as will be understood by those in the art. Generally, when working with ductile metallic powders the process produces good uniform and dense compaction in cylindrical tubes of uniform annular wall thickness sizes ranging from about 0.375 inch to about 1.0 inch. Moreover, the invention can be employed for production of tubular compacts with other cross-sectional shapes, e.g., elliptical, rectangular, hexagonal and square configurations. It is to be especially noted that the invention provides for compaction between rigid surfaces and thus enables close control over dimensional tolerances.

Certain advantageous modes of carrying the invention into practice are illustrated in the accompanying drawing when taken in conjunction with the following description. Referring now to the drawing, FIGS. 1, 2 and 3 illustrate compaction of metal powder by a process according to the invention using apparatus comprising hollow cylindrical mold assembly 10, cylindrical core 11, forwardly tapered frustoconical mandrel l2 and sizing bar 13. The mold assembly comprises mold shells l4 and I5, forward-end expansible seal 16, forward-end retainer clamp 17, rearward-end expansible seal I8 and rearward-end retainer clamp 19. The transverse cross sections, i.e., cross sections perpendicular to the length of core II, of the core, mandrel and sizing bar are circular and the transverse cross sections of the mold assembly components are annular. Shells l4 and 15 are mated along the plane which passes through the longitudinal axis of core 11 and is perpendicular to the drawing. Thus the mated shells form an annular cylinder. FIG. I shows the mold assembly with the rearward seal and retainer clamp removed to enable filling the mold and with the core held in position by locator pins 20 and 21. In the present instance the expansible seals are of rubber. Alternatively, other resilient materials, such as silicone rubber compounds, can be used for the seals. Tubular cavity 22, which is formed by inside face 23 of the mold shells and by core surface 24 and which is closed at one end by seal 16, (the other end being defined by the rearward ends of shells l4 and 15) is filled with powder 25. During filling, the mold can be rapped, tamped, thumped or vibrated, e.g., vibrated by vibrator 26, to obtain advantageously uniform packing of the powder. After filling, the locator pins are removed and the cavity is closed with rearward-end flexible seal 18 and rearward-end retainer clamp 19 as illustrated in FIG. 2. After filling and closing, the mold core 11 is pulled forwardly and can also be rotated, as indicated by arrows on the mandrel in FIGS. 2 and 3, by pulling and rotating means 27. In doing so, the tapered mandrel attached to the core is also rotated and is pulled into the mold cavity, thereby continuously and progressively applying outwardly and forwardly directed pressure on the powder and compacting the powder as illustrated in FIG. 2. Rotation of the mandrel while it moves through the powder is advantageous for obtaining good concentricity by radial distribution of frictional forces. FIG. 2 shows that the transverse cross sections of concave, interior, tube surface 28 are enlarged during passage of the mandrel through the mold while the transverse cross sections of convex, exterior, tube surface 29 are maintained essentially constant during movement of the mandrel through the mold. The mold shells and the mandrel are very rigid, being made of stiff metals such as high-strength steel, and thus undergo only very slight or no distortion during the process. Also, the length of the mass remains essentially constant inasmuch as the seals are rigidly supported by the retainers, which also bind together shells l4 and I5. Fig. 3 shows mold 10 with the mandrel having passed completely through and with the sizing bar moving through the mold so as to more precisely fonn the powder mass to the desired configuration. The sizing bar is of uniform circular cross section with the diameter about the same as, or about 0.010 inch less than, the maximum diameter of the mandrel. After the sizing bar is pulled out of the mold, the mold is disassembled and powder compact 30, which is thus produced by this illustrative process and illustrated in FIG. 4, is removed from the mold. The self-supporting tube compact has adequate green strength to be handled after removal from the mold prior to sintering.

In conjunction with FIGS. 5 and 6, and also FIGS. 1 through 3, it is noted that an expansible sleeve can be provided in the mold and expanded by passage of the mandrel or other progressive expansion device. Also, a liner can be provided in the mold, thus facilitating removal of the compact and enabling additional, and more selectively variable, control over the outside diameter of the compact. FIGS. 5 and 6 show expansible sleeve 31 around mandrel 12 in mold assembly I0 and with liner 32 in the mold. The expansible sleeve can be a split sleeve of a rigid material, such as spring steel or stiff resilient plastic, e.g., sleeve 31 is a split sleeve made of spring steel, that will closely conform to the surface of the mandrel when it passes through the sleeve. As illustrated in FIG. 6, sleeve 31 is longitudinally split at 33 and liner 32 is longitudinally split at 34. The expansible sleeve is initially around the core rod and remains held in place by the expansible seals, thus being disposed successively around the core, the mandrel and the sizing bar while these three components are pulled through the mold. Such an expansible sleeve affords advantages of eliminating friction between the powder and the mandrel and enables lubrication of the mandrel with protection of the powder from contamination by the lubricant. in addition to providing advantages of decreased power requirements through savings in frictional losses the expansible sleeve also serves as a centering device that aids in maintaining concentricity. Furthermore, the expansible sleeve feature enables producing with the same mandrel a variety of compacts having different interior dimensions. For this object the apparatus is provided with a number of interchangeable expansible sleeves which are of different thicknesses, and which can also differ in exterior configuration, and the sleeves are interchanged according to the interior size and/or shape that is to be imparted to the compact.

Where it is not desired to rotate the mandrel, centering contactors, e.g., vanes or pins, can be provided on the core rod in front of the mandrel in order to aid in accurately locating the mandrel in the mold. FIG. 7 shows centering vanes 35, 36 and 37 attached to core 11 and in longitudinally slideable contact with shells 14 and 15 in mold assembly 10, which is illustrated without the powder. Vanes 35, 36 and 37 are disposed radially at angles of 120 from each other around the core. Expansible seal 38 which closes the forward end of the mold is penetrable by the contactors, e.g., seal 38 is made of a soft resilient material such as soft rubber. In operation, the centering contactors slide forwardly along the mold wall when the core is drawn through the mold and thus the contactors guide the core and the mandrel accurately through the center of the mold. When .the contactors are drawn through the forward end of the mold, the contactors slice through penetrable expansible seal 38. To facilitate penetrating the seal, the leading portions of the contactors can be provided with hard and sharp cutting edges. The penetrable seal is held in place, during compaction, by retainer ring 39 and is usually replaced after each use of the mold.

While the mandrel is being moved through the powder mass, ultrasonic vibrations can be applied to the mandrel in order to reduce friction losses, decrease power requirements and aid in insuring a smooth, chatter-free inside surface.

In production of metal tubing, a green compact produced in accordance with the invention is sintered to increase the density thereof and is then mechanically worked, e.g., by drawing through a die or by tube reducing with swagging in a tube reducing machine, to form the sintered compact into wrought tubing. FIG. 8 illustrates cold drawing of sintered powder compact 40 through die 41 so as to reduce the diameter and wall thickness of the tubular form and also densify the compacted and sintered metal to essentially full density metal in drawn tube portion 42.

Where a tapered mandrel is employed in compacting in accordance with the invention, the mandrel has a forward taper such that the transverse cross-sectional area of the forward end is less than that of the rearward end. The forward taper is thus a longitudinally oblique slope. Average forward slope (or taper) of the mandrel surface is usually at least about 0.04 inch per foot (in./ft.) and not greater than about 1.25 in./ft., i.e., a forward slope of about 0.33 percent to about 10.4 percent. For compacting ductile metal powders in tubular masses having initial wall thicknesses of about 0.375 inch to about 1.0 inch, it is advantageous that the forward slope, e.g., the mandrel taper, along which pressure is applied be about 0.04 in./ft. to about 0.375 in./ft., in order to obtain advantages of uniformly dense compaction, low friction, rapid production and maximum utilization of available power. During compaction the wall thickness of the powder mass is usually reduced about 10 percent to about 40 percent, advantageously 20 percent to about 30 percent, for obtaining particularly good strength and density in the green compact. Expansion of the tubular interior should be sufficient to exert the equivalent of at least about 10,000 p.s.i. outward pressure at the interior of the compact. Advantageously the process progressively applies about 30,000 p.s.i., or higher, outward pressure to obtain advantageously high dense compaction of strong ductile powders, e.g., carbonyl nickel powder. Use of a tapered mandrel in expanding the cross sections of the internal surface of the tubular mass to thereby compact the powder provides advantageously good close control over the amount, rate and progress of compaction of the powder and provides good interparticle movement that is beneficial to obtaining dense and uniform compaction. Rate of movement of the mandrel through the powder during compaction can be up to about 30 feet per minute, (ft./rnin.), e.g., about 5 ftJmin. to about 20 ft./min., or higher and thus the invention provides advantages for rapid efficient compaction or long tubes. Continuously progressive compaction in accordance with the invention, such as is obtained when employing a mandrel as illustrated and referred to herein, enables obtaining special advantages over isostatic pressing methods, which advantages include production of long tube shells, uniform wall thickness and economic factors, such as low cost of apparatus and rapid production rates.

It is to be understood that the forward progression of the compacting pressure is applied in the present process through the relative movement of the compacting surface in relation to the tubular mass of powder. Accordingly, the process can also be performed with the sloped compacting surface being held in longitudinally fixed position and with the mold containing the powder mass being moved toward the rearward end of the compacting surface. Moreover, when the sloped compacting surface is entirely of circular transverse cross section, e.g., the curved surface of a frustum of a right cone, either the compacting tool or the powder mass or both can be rotated. For obtaining accurate progress of the compacting tool and for maintaining longitudinally straight alignment of the compacting tool it is highly beneficial to maintain tension on the compacting tool while progressively compacting the powder. How ever, regardless of whether the progressive expansion of the interior cross section of the tubular mass, and thus the compaction of the powder, in the present invention is accomplished with a tapered mandrel or with other apparatus and of whether the compacting tool or the powder mass or both are moved, the expansion is accomplished with pressure applied both forwardly and outwardly from a relatively forwardly moving, forwardly and inwardly sloping, rigid surface and all points along the entire interior perimeter of any given transverse cross section that is being expanded are pressed outward simultaneously.

For the purpose of giving those skilled in the art a better understanding of the invention the following illustrative example is given.

EXAMPLE A hollow cylindrical mold with a cylindrical core positioned axially in the mold was filled with commercially pure carbonyl nickel powder having a Fisher subsieve size of about 5 microns. No binder material was used with the powder. The core was 1.400 inches in diameter and the inside diameter of the mold was 2.625 inches. The forward end of a frustoconical mandrel was fixed to the rearward end of the core, the cross section of the forward end of the mandrel being mated with and the same size as the rear end of the core. The mandrel had a forward linear taper of about 0.171 inch per foot and was of circular cross section with a maximum diameter of 1.742 inch. A sizing bar of uniform circular cross section and about iinches long was incorporated as an integral part of the mandrel, the cross section of the sizing bar being the same as the cross section of the larger (rear) end of the mandrel. The end of the mold was closed with a flexible seal and a retainer clamp. The core, the tapered mandrel and the sizing bar were then pulled forwardly in the mold to thus compact powder in the mold and provide a metal powder compact having dimensions of about 1.74 inch inside diameter, about 2.62 inch outside diameter and at least about 70 inches length. The thus produced green compact had a highly satisfactory uniform density of about 60 percent. The green compact was presintered in the mold for about 1 hour at 1,400 F. in an atmosphere of 88 percent nitrogen and I2 percent hydrogen. The compact was then removed from the mold and given a final sintering treatment for about 8 hours at about 2,200 F. in an 88 percent nitrogen 12 percent hydrogen atmosphere. After final sintering the compact had a density of better than 90 percent of the theoretical density of pure nickel and was satisfactory for cold drawing to form wrought nickel tubing.

It is particularly notable that the sintered compact of the foregoing example was characterized by uniform sintered strength, which confinns that uniform green compaction was obtained when the powder was compacted with the mandrel.

The present invention is particularly applicable in the production of green powder metallurgical compacts which are useful for producing tubing by sintering and drawing the compacts, e.g., the invention is applicable in production of nickel tubing for transmitting corrosive fluids. In addition, the invention is also applicable for production of powder compacts that are useful in the pressed and sintered condition, e.g., articles such as porous bearings, and is highly satisfactory for making self-supporting green compacts without need for having a binder in the powder.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A process for compacting a mass of metallic powder comprising filling a tubular cavity with a metallic powder mass to provide an inner tubular surface and an exterior tubular surface, closing the ends of said tubular cavity to closely confine the powder mass within the cavity, continuously progressively expanding the cross-sectional areas enclosed by said inner surface from one end of said cavity to the other end of said cavity with pressure applied slightly forwardly and mainly outwardly from a relatively forwardly moving rigid surface while maintaining essentially constant the cross-sectional areas enclosed by said exterior tubular surface and also maintaining essentially constant the length of the powder mass in said tubular cavity.

2. A process as set forth in claim 1 wherein the forward and outward pressure is applied along a forward slope in the range of about 0.04 inch per foot to about L25 inch per foot.

3. A process as set forth in claim 1 wherein the powder mass is sintered after expansion of the internal cross section.

4. A process as set forth in claim 3 wherein the sintered powder mass is mechanically worked to reduce the cross-sectional areas between the inner tubular surface and the exterior tubular surface. I

5. A process as set forth in claim 4 wherein the sintered powder mass is mechanically worked to reduced cross-sectional area then drawn to a tube.

6. A process for making a tubular-shaped powder metallurgical compact having a longitudinally uniform interior cross section comprising confining amass of metallic powder in a tubular mold having a rigid inwardly facing mold wall a, core forming an interior passage through the powder mass and two rigidly supported expansible seals between the mold wall and the core, said mold wall and said seals being adapted to prevent outward and lengthwise movement 0 the powder,

moving the core through the interior passage in the powder mass and, immediately following the core, moving a rigid-surfaced forwardly tapered mandrel having a transverse crosssectional area greater than the transverse cross-sectional area of the interior passage formed by the core through the interior passage in the tubularly confined mass to exert outward pressure against the powder surface at the. interior passage and thereby enlarge the transverse cross section of the interior passage and compact said mass between the mandrel and the rigid interior wall of the mold into a tubular form having a longitudinally uniform interior cross section while retaining the powder in the mold and maintaining the length of the mass essentially constant.

7. A process as set forth in claim 6 wherein the forward taper of the mandrel is in the range of about 0.04 inch per foot to about 1.25 inch per foot.

8. A process as set forth in claim 6 wherein the powder mass is connected uniformly into a tubular-shaped configuration having a length-to-wall thickness ratio of at least 5:1.

9 A process as set forth in claim 6 wherein the tubular mass is compacted an amount equivalent to a 10 percent to 40 percent reduction in the wall thickness of the tubular mass.

10. A process as set forth in claim 6 wherein the mandrel is of circular cross section and is rotated around the axis of said circular cross section while being moved through the powder mass.

11. A process as set forth in claim 6 wherein the mandrel is vibrated while being moved through the powder mass.

12. A process as set forth in claim 6 wherein the powder mass is sintered after compaction.

13. A process comprising sintering a tubular-shaped compact produced by the process set forth in claim 6 and thereafter mechanically working the sintered tubular compactto reduce the wall thickness thereof and form the compact into sintered and wrought tubing.

* l l l Column 5, line 75 gggy UNITED STA'IES IATENT OFFICE CERTIFICATE OF CORRECTION Patent No. ,3 Dated October 26, 1971 Inventor) Charles E. Manilla, Franklin C. Kelly and Richard H. Hanewald It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

, for "to about 0.375 in./ft.,

read --to about 1.25 in./ft., more advantageously about 0.12

in./ft. to about 0,375 in./ft.,--.

Column 6, line 22, for "or" read --of--.

Column 8, line 3?, Claim 8 for "connected" read --compacted--.

Signed and sealed this 27th day of June 1972.

(SEAL) Attest:

EDWARD M.FLETCHER, JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

2. A process as set forth in claim 1 wherein the forward and outward pressure is applied along a forward slope in the range of about 0.04 inch per foot to about 1.25 inch per foot.
 3. A process as set forth in claim 1 wherein the powder mass is sintered after expansion of the internal cross section.
 4. A process as set forth in claim 3 wherein the sintered powder mass is mechanically worked to reduce the cross-sectional areas between the inner tubular surface and the exterior tubular surface.
 5. A process as set forth in claim 4 wherein the sintered powder mass is mechanically worked to reduced cross-sectional area then drawn to a tube.
 6. A process for making a tubular-shaped powder metallurgical compact having a longitudinally uniform interior cross section comprising confining a mass of metallic powder in a tubular mold having a rigid inwardly facing mold wall a, core forming an interior passage through the powder mass and two rigidly supported expansible seals between the mold wall and the core, said mold wall and said seals being adapted to prevent outward and lengthwise movement of the powder, moving the core through the interior passage in the powder mass and, immediately following the core, moving a rigid-surfaced forwardly tapered mandrel having a transverse cross-sectional area greater than the transverse cross-sectional area of the interior passage formed by the core through the interior passage in the tubularly confined mass to exert outward pressure against the powder surface at the interior passage and thereby enlarge the transverse cross section of the interior passage and compact said mass between the mandrel and the rigid interior wall of the mold into a tubular form having a longitudinally uniform interior cross section while retaining the powder in the mold and maintaining the length of the mass essentially constant.
 7. A process as set forth in claim 6 wherein the forward taper of the mandrel is in the range of about 0.04 inch per foot to about 1.25 inch per foot.
 8. A process as set forth in claim 6 wherein the powder mass is connected uniformly into a tubular-shaped configuration having a length-to-wall thickness ratio of at least 5:1. 9 . A process as set forth in claim 6 wherein the tubular mass is compacted an amount equivalent to a 10 percent to 40 percent reduction in the wall thickness of the tubular mass.
 10. A process as set forth in claim 6 wherein the mandrel is of circular cross section and is rotated around the axis of said circular cross section while being moved through the powder mass.
 11. A process as set forth in claim 6 wherein the mandrel is vibrated while being moved through the powder mass.
 12. A process as set forth in claim 6 wherein the powder mass is sintered after compaction.
 13. A process comprising sintering a tubular-shaped compact produced by the process set forth in claim 6 and thereafter mechanically working the sintered tubular compact to reduce the wall thickness thereof and form the compact into sintered and wrought tubing. 